This invention relates to holographic displays for projecting head-up images onto a vehicle windshield.
A floodlit holographic telltale display, as described in U.S. Pat. Nos. 4,790,613 and 4,795,223 is capable of producing a set of bright, uniform, multi-colored virtual images in the plane of the windshield. In U.S. Pat. No. 4,790,613 the holograms are illuminated by the output from optical fibers.
The holographic telltale display has two disadvantages. It suffers from ambient turn-on, which is caused by light from the sun passing through the telltale hologram, having 4% reflection off the lower windshield glass/air interface, and illuminating the telltale hologram which diffracts the light to the driver. In addition, objectionable virtual images of the light source or holes cut through the dashboard to pass the light to the windshield case can be reflected off the windshield.
Ideally, the car manufacturers want to make the dash featureless except for the uniform corrugated visual texture of the dashboard material. Putting holes in the dash for the light sources create a discontinuity in the veiling glare brightness pattern.
Therefore, it would be advantageous to provide a display which had all of the advantages of the floodlit telltale display but without the disadvantages described above.